In some embodiments of EUV light sources, a monitoring laser beam for testing an alignment of optical components is reflected at a planar plate aligned at an angle to the monitoring laser beam and registered by a detector. The wavelength of the monitoring laser beam differs from the wavelength of a further laser beam (e.g., a CO2 laser beam) that is transmitted by the planar plate, which forms a window in a vacuum chamber.
A small portion of radiation power incident on the planar plate (e.g., which has a tilt) that has been introduced into a beam path is typically reflected to a location outside of the beam path, even in the case of a laser beam that is transmitted by the planar plate. For example, the laser radiation is not only reflected back from one side of the planar plate, but rather, each side of the planar plate reflects a partial beam of the incident laser beam. It has been found that interference strips emerge in recorded images when monitoring or analyzing a laser beam by observing the laser radiation reflected at a transmissive optical element on a spatially resolving detector (e.g., a camera), and so only a few details of a beam cross section of the laser beam to be monitored and imaged on the detector may still be identifiable.
The difference between the degrees of reflection of the two sides of the planar plate can be increased by a reflecting coating applied to one of the sides in order to avoid such interference strips. However, the reflectivity of such a coating should not be selected to be too high, particularly in the case of laser beams with a high laser power (e.g., of multiple kilowatts), such as laser beams generated by a driver laser arrangement. Moreover, a partial beam reflected at one of the two sides, the power of which only has a few percent of the power of the partial beam reflected at the other side, can already lead to clearly visible interference strips. Alternatively, attempts can be made to remove the interference strips in the recorded images with the aid of numerical image processing algorithms, but this technique generally does not adequately remove the interference strips.
In some embodiments, a device for focusing a laser beam on a workpiece includes a transmissive optical element in the form of a planar plate that is arranged at a tilt angle in relation to a beam axis of the laser beam in a convergent beam path of the laser beam and includes a spatially resolving detector for registering laser radiation reflected back at the transmissive optical element. Assigned to the detector are means for distinguishing laser radiation reflected back at a first side of the optical element from laser radiation reflected back at a second side of the optical element. The means can be an image evaluation apparatus or a diaphragm that masks the laser radiation reflected back from one of the sides of the transmissive optical element. Methods for monitoring laser processing may be associated with such a device.
In certain embodiments, methods exist for coaxial beam analysis at optical systems in which a defined percentage of a beam is reflected back coaxially or at a small angle at an optical face that is perpendicular to a chief ray. The partial beam reflected back is separated from the chief ray by a beam splitter, where the partial beam is available for beam analysis. An optical element with a back-reflecting surface can have a wedge angle in order to be able to separate the reflection of the two surfaces on the image side.